3 Answers
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Not my region (lavender is mostly found in Provence, but there are occasional fields in other regions), so I need to rely on Wikipedia. Lavender flowers from late June to late August. It is typically harvested well before the flower wilts, because the heat makes the essence rise towards the flower's glands, so the plant's aroma becomes less strong as the summer goes by. The exact harvest time varies from region to region (and of course with the year's weather); looking around on the Internet, I see dates ranging from mid-July to mid-August; for example there are lavender festivals in Provence from the third Sunday in July to the 15th of August; the harvest tends to be earlier in the Drôme region.

Dutch wikipedia about Lavender says they have flowers from August till October. And because the best chances for nice weather are during the summer (which would improve the experience), I would suggest to go somewhere around the middle of August.

It actually says that the plant should be pruned “after the flowering (August to October)”. The sentence is somewhat ambiguous but it is apparently the time you can prune it. There will perhaps still be flowers visible in gardens in the Netherlands but it's definitely too late for the fields in the south of France. Besides, weather is really not a problem in this region, there is plenty of sun in June-July. If anything, August can be less comfortable (too hot, with some thunderstorms).
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RelaxedMay 17 '13 at 22:51

This will be too late for the original poster but here is a map. Apparently, in many places the flowers are already gone by the end of July.

I would say that the best period is the beginning of July. It's also the time when lavender honey is collected. I never attended one of the late August festivals but my understanding is that they take place around the time when the harvest finishes so there would probably be less to see in the fields.